Add alpine support #17

Job	Run time
build-and-test (debian, linux/amd64)	14m 17s
build-and-test (debian, linux/i386)	2m 26s
build-and-test (debian, linux/arm/v5)	18m 49s
build-and-test (debian, linux/arm/v7)	17m 54s
build-and-test (debian, linux/mips64le)	18m 7s
build-and-test (debian, linux/ppc64le)	18m 51s
build-and-test (debian, linux/s390x)	16m 52s
build-and-test (debian, linux/arm64)	2h 30m 14s
build-and-test (alpine, linux/amd64)	18m 43s
build-and-test (alpine, linux/i386)	2m 10s
build-and-test (alpine, linux/arm/v6)	14m 38s
build-and-test (alpine, linux/arm/v7)	14m 43s
build-and-test (alpine, linux/ppc64le)	16m 24s
build-and-test (alpine, linux/s390x)	16m 6s
build-and-test (alpine, linux/arm64)	2h 29m 11s
build-and-test (alpine, linux/riscv64)	12m 58s
	8h 22m 23s

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